The present application claims priority of Japanese Patent Application No. JP2002-042478, filed in Japan on Feb. 20, 2002, the entire contents of which are hereby incorporated herein by reference.
1. Field of the Invention
The present invention relates to a projection encoder based on a triple-grating concept involving the use of a semiconductor substrate or other type of substrate with transmission gratings and photodetectors, and more particularly to a miniature, compactly structured projection encoder capable of accurately generating a home position signal for positional detection.
2. Description of the Related Art
The inventor et al have already proposed, in JP-A 2000-321097 and elsewhere, a projection encoder based on a triple-grating concept. This projection encoder comprises an LED as a light source, a movable plate composed of a semiconductor substrate on which transmission gratings and photodetectors (photodiodes) are integrally formed at a constant pitch, and a reflecting grating plate on which reflecting gratings are formed at a constant pitch; and has a structure in which the movable plate is disposed between the LED and the reflecting grating plate.
In the projection encoder with this structure, the movable plate is moved integrally with the measurement object in the direction in which the transmission gratings and photodiodes are arranged along the direction perpendicular to the optical axis of light emitted by the LED. Light emitted by the LED first strikes the back surface of the movable plate, passes through the transmission gratings on the movable plate, and forms grating stripes on the surface of the reflecting grating plate. Reflecting gratings are also formed at a constant pitch on the reflecting grating plate, making it possible to reflect only those components of light striking the reflecting grating plate that irradiate the reflecting gratings. The reflecting grating image is caused to re-irradiate the movable plate, and is received by photodiodes shaped as vertical stripes and formed at a constant pitch and a constant width.
The photodiodes and transmission gratings shaped as vertical stripes on a movable grating plate function as two grating plates. Consequently, the amount of light received by a photodiode is converted, based on a triple-grating concept involving the use of reflecting gratings, to a sinusoid waveform in accordance with the relative movement of the reflecting grating plate and the movable grating plate. It is thus possible to obtain a pulse signal that corresponds to the relative speed of travel and is based on the photocurrent of the photodiodes, and to calculate the relative speed of travel on the basis of the pulse rate of the pulse signal.
In addition, arranging the photodiodes such that an A-phase signal and a B-phase signal differing in phase by 90 degrees can be obtained makes it possible to determine the movement direction of the movable grating plate on the basis of these two-phase signals.
Thus, the transmission gratings and photodetectors of the projection encoder disclosed in the above laid-open publication and elsewhere are fabricated using semiconductor manufacturing technology, making it possible to manufacture fine-pitch gratings and to obtain high-resolution encoders. In addition, the photodetectors formed as vertical stripes at a constant pitch function as a grating, and the grating itself has a lens effect, so an optical lens system can be dispensed with and the device can be miniaturized. Another feature is that the use of the triple-grating concept prevents resolution from being adversely affected by the width or variability of gaps in such reflecting gratings and transmission gratings, so the adjustment operations needed to ensure the required mounting accuracy for the members with these gratings can be simplified, and the restrictions imposed on the mounting locations can be eased. In addition to this, the interval between the reflecting gratings and transmission gratings can be increased, making it possible to obtain benefits such as achieving better environmental resistance by, for example, housing the reflecting gratings in a protective casing.
A home position, which constitutes the basis for a position or rotation angle, must be detected herein in order to determine the relative position of the movable plate and the reflecting grating plate on the fixed side. Providing the movable plate with integrally formed photodetectors for detecting the home position, and providing the reflecting grating plate with reflecting gratings for detecting the home position have been proposed as a means of detecting the home position. In this case, the contrast of a reflected light image for detecting the home position decreases and the level of the detection signal becomes lower when a wider interval is maintained between the movable plate and reflecting grating plate. As a result, signals for detecting the home position become blocked out by noise, and there is the danger that the home position will be impossible to detect with sufficient accuracy.
It has been proposed to increase the number of reflecting gratings and photodetectors for origin detection, and to use differential detection for the home position in order to stabilize and raise the level of detection signals. However, when an attempt is made to merely use differential detection, the result is an increase in the size of movable plates and reflecting grating plates. In other words, it is impossible to accurately detect a home position when restrictions are imposed on the size of movable plates and reflecting grating plates because it is impossible in this case to increase the number of the photodetectors and reflecting gratings for detecting the home position.
In view of the above, an object of the present invention is to provide a projection encoder that can accurately detect the home position and is based on the triple-grating concept.
Another object of the present invention is to provide a projection encoder that has a small-sized home position detection mechanism, is compactly structured, and is based on the triple-grating concept.
Aimed at attaining the stated objects, according to an embodiment of the present invention, there is provided a projection encoder, which has a light source, reflecting gratings having a prescribed shape and arranged at a constant pitch, transmission gratings having a prescribed shape and arranged at a constant pitch, and photodetectors for receiving a reflected light image that is emitted by the light source, transmitted by the transmission gratings, and reflected by the reflecting gratings, with at least the relative movement position of the reflecting gratings and transmission gratings being detected based on a detection signal obtained from the photodetectors, comprising:
a reflecting grating plate on which the reflecting gratings are formed, a substrate on which the transmission gratings and the photodetectors are formed, and a home position detection mechanism for detecting a home position of the reflecting grating plate and the substrate; wherein
the home position detection mechanism has
a reflecting grating region designed for detecting the home position and formed on the reflecting grating plate; and
a photodetector region designed for detecting the home position and formed on the substrate; and wherein
nonreflecting gratings and reflecting gratings of constant width are arranged in accordance with an M-series arrangement pattern in the reflecting grating region for detecting the home position;
Z-phase photodetectors and Zxe2x80x2-phase photodetectors of constant width for obtaining Z-phase signals and Zxe2x80x2-phase signals differing in phase are arranged in accordance with the M-series arrangement pattern in the photodetector region for detecting the home position; and
a positional relation of the reflecting grating region for detecting the home position and the photodetector region for detecting the home position is set in a manner such that the reflected light image reflected by the reflecting grating region for detecting the home position can be received in the photodetector region for detecting the home position.
Adopting an M-series arrangement pattern causes the difference between the amount of light received by a Z-phase photodetector and the amount of light received by a Zxe2x80x2-phase photodetector to be at a maximum when there is a match between the arrangement pattern in the reflecting grating region for detecting the home position and the arrangement pattern in the photodetector region for detecting the home position, and brings the difference between the amounts of light substantially close to zero when the two arrangement patterns do not match. Consequently, a home position signal with a high S/N ratio can be consistently produced by taking the difference in the detection signals from the Z-phase photodetectors and Zxe2x80x2-phase photodetectors.
In this arrangement, it is possible to form on the substrate a transmitting region in which the transmission gratings are arranged at a constant pitch, a light-receiving region in which the photodetectors are arranged at a constant pitch on one side of the transmitting region, and a photodetector region for detecting the home position in which the Z-phase photodetectors and Zxe2x80x2-phase photodetectors are disposed at a constant pitch on the other side of the transmitting region. In this case, the reflecting grating plate should be provided with a reflecting grating region in which the reflecting gratings are arranged at a constant pitch in positions that can be disposed facing the light-receiving region, and a reflecting grating region for detecting the home position in which the reflecting gratings and nonreflecting gratings are arranged at a constant pitch in positions that can be disposed facing the photodetector region for detecting the home position.
In addition, the reflecting gratings and nonreflecting gratings arranged at a constant pitch in the reflecting grating region designed for detecting the home position and formed on the reflecting grating plate should be composed, respectively, from bisected reflecting gratings and bisected nonreflecting gratings that have half the width and are disposed at half the pitch in order to enhance the contrast of the reflected light image for detecting the home position.
The home position detection mechanism in the projection encoder of the present invention further comprises a signal processing unit for detecting the home position on the basis of the differential signal between a detection signal produced by the Z-phase photodetectors and a detection signal produced by the Zxe2x80x2-phase photodetectors.